Trenchless boring technology has been developed for drilling a horizontal bore underground for passage of a utility line or the like, without the need to excavate the entire length of the line. In the usual process, a drill rig will first drill a pilot bore. It is essential to have a steerable boring tool which permits control of the direction of boring to insure that the pilot bore will end at the desired location.
After the pilot bore has been completed, a back reamer is run through the pilot bore, typically in the direction opposite the direction that the pilot bore was formed. The back reamer expands the bore to the desired final diameter and compacts and stabilizes the earth forming the walls of the bore to resist collapse of the bore.
A back reamer with a flighting, or helical screw style of thread, is typical of prior back reamer design. The flighted back reamer is rotated while it is pulled through the pilot bore to screw into the compactable soil. Straight cones have also been used, which are pulled without rotation through the pilot bore, although these cones require significant pullback forces.
Fluid has also been injected in the pilot bore to enlarge the bore. This fluid injection creates a slurry that must be partially removed from the borehole before the utility line is placed through the bore.
A need yet exists for an improved process and apparatus for drilling a pilot bore and back reaming the pilot bore with the minimum expenditure of energy while insuring a stable and well compacted bore wall.